Heretofore, thermo-curable resins having an imide structure have been widely used in industrial fields, due to their excellent physical properties such as electric insulation, heat resistance and an ability to provide the resulting molded products with dimensional stability.
Thermo-curable resins prepared from aromatic bismaleimides are materials which are insoluble and unmeltable, as well as excellent in heat resistance. However, they have some disadvantages such as poor processability, impact resistance and stiffness.
In order to improve impact resistance and stiffness of such aromatic bismaleimide-based resins, there has been proposed a method in which an aromatic diamine is used in combination with the aromatic bismaleimide. For example, a polyaminobis-maleimide resin ("Kynnel"; produced by Rhone Poulenc S. A) composed of N,N'-diphenylmethane bismaleimide and 4,4'-diaminodiphenylmethane is better in impact resistance and stiffness than a resin composed of an aromatic bismaleimide alone, and therefore its molded products have been widely used as structural materials, slider materials, and so on. However, even such thermo-curable resins are far from satisfactory in regards to impact resistance and stiffness.
On the other hand, in general, as printed circuit boards for electrical and electronics systems, paper base phenolic resin laminates in which paper is used as a substrate and glass fabric base epoxy resin laminates in which glass cloth is used as a substrate have been mainly used. However, the increase in heat release value has been a large problem with trends toward increasing packaging density and higher densities of circuit patterns. Therefore, improvement of heat resistance of such substrates has become an important subject.
For the purpose of improving heat resistance, bismaleimide-based resins have been studied as resins for impregnation, in the place of epoxy-based resins. Generally, compared with epoxy resin laminates, bismaleimide-based resin laminates are superior in heat resistance, and therefore have been rewardingly used for multilayer printed circuit boards. However, such laminates have a room for improvement in the adhesiveness of layers.
On the other hand, in case of condensation-type polyimide-based resins, although the expected electrical properties and mechanical properties are excellent, a disadvantage that condensed water given during imidization lowers the adhesiveness and heat stability and generates voids still remains.
In methods for producing copper-clad laminates, in case of copper-clad laminates of phenol resin using paper as a substrate, a phenol resin-based adhesive generally used in a pre-coated form on a copper foil. On the other hand, in case of copper-clad laminates of epoxy resin using a glass cloth as a substrate, a copper foil with no adhesive coating is specially used.
Further, in case of copper-clad laminates of bismaleimide-based resins or condensed-type polyimide-based resins, an epoxy resin-based adhesive is generally used in a pre-coated form on a copper foil.
However, such phenol resin-based and epoxy resin-based adhesives have a defect in that they exhibit poor heat resistance.
Accordingly, the object of the present invention is to provide a thermo-curable resin composition in which strength is not lowered by exposure to high temperatures for a long time and which has excellent electric properties, processability, impact resistance and stiffness, and a method for producing a copper-clad laminate using the composition as an adhesive, as well as a method for producing a copper-clad laminate using the composition as a resin for impregnation, comprising laminating a copper foil with no adhesive coating.